Studies of orthodontic elastomeric modules. Part 1: Glass transition temperatures for representative pigmented products in the as-received condition and after orthodontic use

Abstract

The purpose of this study was to investigate the glass transition temperatures (T g) of representative elastomeric chain products (plastic modules) in the as-received condition and after orthodontic use to determine differences between brands and pigments. Values of T g were determined by differential scanning calorimetry. Products were obtained from 3 manufacturers: Rocky Mountain Orthodontics (RMO, Denver, Colo), Ormco (Glendora, Calif), and G&H (Greenwood, Ind). Three colors (gray, red, and purple) were selected for each brand-pigment combination to evaluate the as-received products, and test specimens for each brand-pigment combination from the same batches were placed in patients' mouths for 4 weeks to evaluate the products after clinical use (sample size of 7 specimens for both groups of experiments). Results were analyzed statistically by multiple nonparametric Mann-Whitney tests. A post hoc step-down Bonferroni analysis followed, to examine differences in T g due to pigmentation within brands and differences due to brands within each pigmentation category. For the as-received products, the RMO modules had mean T g ranging from −24°C to −21°C, whereas the Ormco and G&H modules had significantly lower mean T g, ranging from −46°C to −39°C, indicating substantial compositional or polymer structural differences compared with the RMO modules. After clinical use, the RMO products had mean T g ranging from −31°C to −25°C, whereas the Ormco and G&H products had mean T g ranging from −46°C to −30°C. The in vivo specimens also exhibited a second, higher-temperature glass transition of unknown origin. Results suggest that the Ormco and G&H products should have greater flexibility than the RMO products and that there should be significant differences in clinical force-degradation behavior for the Ormco and G&H products compared with the RMO products.